In the design of conventional operational amplifier circuitry utilizing CMOS technologies, it has been known that higher operational voltages are desirable. The higher operating voltages provide low output impedance per device size, greater dynamic range and avoidance of early clipping.
Higher voltages have been limited in such circuitry due to the low field threshold turn-on voltages of the transistors resulting from field parasitics. Also, higher doping on the drain junctions forces lower break-down from drain-to-source and drain-to-bulk. A bipolar action usually takes place from the higher field around the drains, which limits the operating voltages, and forces narrow design constraints.
In many other circuit applications, it is necessary to employ both low supply and high supply sources.
To improve the speed of audio processing chips (CODECS), it is desirable to use small transistors (in the sub-micron region) to gain both speed and density. But for the on chip analog-to-digital and digital-to-analog conversion, higher voltage swings are desirable to provide a greater dynamic range, avoid early clippings and, to provide a very clean low impedance mid-level ground.
In DMOS devices where threshold voltages are close to 3 volts, a driver with at least 4 times the threshold voltage, i.e., 12 volts, is desired in order to provide low turn-on impedance.
Accordingly, higher voltage MOS transistors have been fabricated by surrounding the drain with a high resistance layer of Al polycrystalline silicon (polysilicon). The high resistance layer prevents field concentration near an edge of a gate electrode.
Such a teaching is presented in U.S. Pat. Nos. 4,614,959, issued to: Nakagawa on Sept. 30, 1986; and 4,766,474, issued to Nakagawa et al. on Aug. 23, 1988.
While the aforementioned shielding is effective for its intended purposes, it nevertheless has some drawbacks:
(a) such devices usually require special doping processes that add to the complexity and cost of manufacture; and PA1 (b) such shielding, while providing higher operating voltages, usually falls short of operation voltage levels above 10 volts, that are required for high performance circuits.